A power amplifier (PA) circuit may be characterized by its mode, or “class” of operation. Exemplary classes include Class A, Class AB, and Class B. In Class A operation, a PA may operate in a conducting, or ON, state during 100% of the cycle, or the entire cycle, of the input signal. In Class A operation, the output signal from the PA is typically a scaled version of the input signal, where the scaling factor is a function of the gain associated with the PA circuit. However, for Class A operation, the PA is typically in a conducting state even when there is no input signal. Furthermore, even when the PA is amplifying an input signal, the efficiency of the PA may not exceed 50%.
In Class B operation, a PA may operate in a conducting state during 50%, or half, of the cycle of the input signal. This may result in large amounts of distortion of the input signal in the output signal. The higher efficiency of the Class B PA results from the PA being in a non-conducting, or OFF, state half of the time.
In Class AB operation, a PA may operate in a conducting state for greater than 50%, but less than 100%, of the cycle of the input signal. In Class AB operation, the PA may be more efficient than in Class A operation, but less efficient than in Class B operation. Furthermore, in Class AB operation, the PA may produce more distortion than in Class A operation, but less than in Class B operation.
When the peak input signal level to a PA circuit is large compared to the average input signal level, or high peak to average ratio, the PA circuit may be biased to accommodate the peak input signal level, PINMAX. The value of PDC may be set to enable generation of an RF signal output level from the PA circuit, PRFMAX, when the corresponding input signal level is PINMAX. Thus, efficiency of the PA circuit may be highest for a given value PDC when the RF signal output level from the PA circuit is PRFMAX. However, for high peak to average ratios, the input signal level is typically less than PINMAX for a substantial portion of the time that the PA circuit is operating. Therefore, the average RF signal output level, PRFAVG, may be significantly lower than PRFMAX. Consequently, the need to support high peak to average ratios may result in low efficiency for the PA circuit.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.